Organic light emitting diodes (OLEDs) are a particularly advantageous form of electro-optic display. They are bright, colorful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) LEDs may be fabricated using either polymers or small molecules in a range of colors (or in multi-colored displays), depending upon the materials used. A typical OLED device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer or a light emitting low molecular weight material, and the other of which is a layer of a hole injecting material such as a polythiophene derivative or a polyaniline derivative.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-color pixelated display. A multi-colored display may be constructed using groups of red, green, and blue emitting pixels. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel while passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image.
FIG. 1 shows a vertical cross-section through an example of a prior art OLED device 100. In an active matrix display part of the area of a pixel is occupied by associated drive circuitry (not shown in FIG. 1). The structure of the device is somewhat simplified for the purposes of illustration.
The OLED 100 comprises a substrate 102, typically 0.7 mm or 1.1 mm glass but optionally clear plastic, on which an anode layer 106 has been deposited. The anode layer typically comprises around 150 nm thickness of ITO (indium tin oxide), over which is provided a metal contact layer, typically around 500 nm of aluminum, sometimes referred to as anode metal. Glass substrates coated with ITO and contact metal may be purchased from Corning, USA. The contact metal (and optionally the ITO) is patterned as desired, and so that it does not obscure the display, by a conventional process of photolithography followed by etching.
A substantially transparent hole injection layer 108a is provided over the anode metal, followed by an electroluminescent layer 108b. Banks 112 may be formed on the substrate, for example from positive or negative photoresist material, to define wells 114 into which these active organic layers may be selectively deposited, for example by a droplet deposition or inkjet printing technique. The wells 114 thus define light emitting areas or pixels of the display.
A cathode layer 110 is then applied by, say, physical vapor deposition. A cathode layer typically comprises a low work function metal such as calcium or barium covered with a thicker, capping layer of aluminum and optionally including an additional layer immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Mutual electrical isolation of cathode lines may achieved through the use of cathode separators. Typically a number of displays are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed, and the displays separated before an encapsulating can is attached to each to inhibit oxidation and moisture ingress.
Organic LEDs of this general type may be fabricated using a range of materials including polymers, dendrimers, and so-called small molecules, to emit over a range of wavelengths at varying drive voltages and efficiencies. Examples of polymer-based OLED materials are described in WO90/13148, WO95/06400 and WO99/48160; examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343; and examples of small molecule OLED materials are described in U.S. Pat. No. 4,539,507. The aforementioned polymers, dendrimers and small molecules emit light by radiative decay of singlet excitons (fluorescence). However, up to 75% of excitons are triplet excitons which normally undergo non-radiative decay. Electroluminescence by radiative decay of triplet excitons (phosphorescence) is disclosed in, for example, “Very high-efficiency green organic light-emitting devices based on electrophosphorescence” M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, and S. R. Forrest Applied Physics Letters, Vol. 75(1) pp. 4-6, Jul. 5, 1999. In the case of a polymer-based OLED layers 108 typically comprise a hole injection layer 108a and a light emitting polymer (LEP) electroluminescent layer 108b. A further hole transport layer (not shown) may be provided between hole injection layer 108a and electroluminescent layer 108b. The electroluminescent layer may comprise, for example, around 70 nm (dry) thickness of PPV (poly(p-phenylenevinylene)) and the hole injection layer, which helps match the hole energy levels of the anode layer and of the electroluminescent layer, may comprise, for example, around 50-200 nm, preferably around 150 nm (dry) thickness of PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene).
A volatile solvent is generally employed to deposit a molecular electronic material (typically an organic semiconducting material), with 0.5% to 4% dissolved solvent material. This can take anything between a few seconds and a few minutes to dry and results in a relatively thin film in comparison with the initial “ink” volume. Often multiple drops are deposited, preferably before drying begins, to provide sufficient thickness of dry material. Solvents which may be used include cyclohexylbenzene and alkylated benzenes, in particular toluene or xylene; others are described in WO 00/59267, WO 01/16251 and WO 02/18513; a solvent comprising a blend of these may also be employed.
FIG. 2, which is taken from WO2005/076386 (hereby incorporated by reference), shows a view from above (that is, not through the substrate) of a portion of a three-color active matrix pixelated OLED display 200 after deposition of one of the active color layers. FIG. 2 shows an array of banks 112 and wells 114 defining pixels of the display. The wells 114 are formed as apertures in a continuous layer or sheet.
Techniques for the deposition of material for organic light emitting diodes (OLEDs) using ink jet printing techniques are described in a number of documents including, for example, Y. Yang, “Review of Recent Progress on Polymer Electroluminescent Devices,” SPIE Photonics West: Optoelectronics '98, Conf. 3279, San Jose, January, 1998; EP O 880 303; and “Ink-Jet Printing of Polymer Light-Emitting Devices”, Paul C. Duineveld, Margreet M. de Kok, Michael Buechel, Aad H. Sempel, Kees A. H. Mutsaers, Peter van de Weijer, Ivo G. J. Camps, Ton J. M. van den Biggelaar, Jan-Eric J. M. Rubingh and Eliav I. Haskal, Organic Light-Emitting Materials and Devices V, Zakya H. Kafafi, Editor, Proceedings of SPIE Vol. 4464 (2002). Ink jet techniques can be used to deposit materials for both small molecule and polymer LEDs.
Precision ink jet printers such as machines from Litrex Corporation of California, USA are used; suitable print heads are available from Xaar of Cambridge, UK and Spectra, Inc. of NH, USA. A typical print head is more clearly in FIG. 3. The print head 222 has a plurality of nozzles 227, typically orifices in a nozzle plate for ejecting droplets of fluid from the print head onto the substrate. A fluid supply for printing (not shown) may either be provided by a reservoir within the print head 222 or print head unit or fluid may be supplied from an external source. In the illustrated example the print head 222 has a single row 228 of nozzles 227, but in other examples of print heads more than one row of nozzles may be provided with nozzles offset in one or two dimensions. The diameter of the orifices of nozzles 227 is typically between 10 μm and 100 μm, and drop sizes are similar. The space or pitch between adjacent nozzle orifices is typically between 50 μm and 100 μm.
Printers using such print heads 222 automatically divide up the print area of the display into a number of swathes and print these in succession as shown schematically in FIGS. 4a and 4b following. This is because a normal display panel has more pixels in its width than a print head can print in a single print pass. For example, some prior art printers have 128 nozzles, but normally only 80 are used in a single print pass.
FIG. 4a is a diagram showing the printing of swathes as known in the art. There is shown a first swathe 10, a second swathe 11 and a third swathe 12, as well as the order in which they are printed. FIG. 4b shows a conventional printing strategy in which print head 222 prints successive swathes 302, 304 in the Y-direction, stepping in the X-direction between each swathe. The technique illustrated in FIGS. 4a and 4b may be employed to produce a finer dot pitch. The print head is positioned at an angle Φ to the X-direction to reduce the dot pitch by a factor of cos Φ. Generally the size or volume distribution of drops is non-uniform, increasing or falling off at nozzles at the edge of the print head (that is, near an end of a row of nozzles), and further non-uniformity arise from small variations in nozzle heights.
Some of the problems with prior art swathe printing are described following:
As previously explained, to deposit a molecular electronic material a volatile solvent such as toluene or xylene is employed with 1-2% dissolved solvent material. This results in a relatively thin film in comparison with the initial “ink” volume. The drying time is dependent upon the solvent mix and the atmosphere above the substrate, but typically varies between a few seconds and some minutes. It is strongly preferable all the drops comprising material which are eventually to make up a pixel are deposited before drying begins.
Solvent drying effects make the appearance of pixels on the edge of a swathe subtly different to those in the centre of a swathe, as drops along the edge of a swathe dry faster and where the drop is thinnest, more light is emitted by the display and a visible line can be seen. The printing of display panels in swathes results in a “striped” appearance within the display.
Such “swathe-edge” problems can be partially alleviated by the use of ink formulations designed to slow down the drying until all swathes are printed, or by tuning the driving of the pixels to drive those at swathe edges differently to other pixels—however both approaches are complex and have their own restrictions.
A second effect is usually caused by a malfunctioning nozzle which either puts down too little, or two much, ink. When depositing materials for molecular electronic devices such as OLEDs, there is a need for both high resolution, generally than better than that required for the best high resolution graphics, and accurate control of the volume of material deposited. For graphics applications it is drop placement that is significant and volume variations of 5 to 10% are acceptable. However, when constructing molecular electronic devices it is deposited “ink” volume which is important since this will determine the eventual film thickness which, for an OLED, impacts upon brightness and hence drive current and device lifetime. Thus it is desirable to achieve a volume variation of better than 2%, preferably better than 1%, across an entire OLED display.
Further, if the volume variation of one column in a swathe is out by more than 5% out compared to result of another, normal, column, this then can be seen by human eye. The result is a visible “swathe-line” that can be seen the length of the swathe and which is repeated in every swathe printed.
Some prior art printers try to mitigate some of these swathe effects by interlacing the printing of swathes in the print pattern. However, the interlacing pattern is a simple on-off pattern where every other pixel is printed in a first print pass and the remaining pixels are filled in a second print pass.
FIG. 4 which is a diagram showing the printing of swathes in an on-off print pattern as known in the art. There is as shown a first print pattern 20 and a second print pattern 21. Also shown is the result of two print passes using each print pattern in sequence, which is a fully filled swathe 22.
This method has been found to actually lead to more swathe-edge effect problems (because in effect there are now more, if not smaller, swathes) and further, swathe-line effects due to faulty nozzles still can not be compensated for.
Interlacing using more complex patterns of pixels positions is not known for the printing of display panels.
It is an aim of certain embodiments of the present invention to provide an improved method of ink jet printing display panels which overcomes, or at least mitigates, swathe-effect problems.
It is a further aim to provide an improved display panel.